This invention relates to a communication system such as a mobile radio system and, in particular, to a terminal station including a receiving apparatus and a transmitting apparatus.
In a conventional communication system, it is possible to use various modulation systems for carrying out communication between the terminal station and a base station in the manner known in the art. One of the modulation systems is a frequency modulation system known in the art. Another of the modulation systems is a digital modulation system known in the art.
A mobile radio system is known as one of the conventional communication systems. The mobile radio system has a plurality of service areas or service zones which are generally adjacent to one another with a part thereof being overlapped to each other. In the mobile radio system, use is generally made of the frequency modulation system. In other words, each of the service areas is assigned with the frequency modulation system.
Recently, digital techniques have become remarkably advanced and widely used in the mobile radio system. Under the circumstances, the digital modulation system is often used in addition to the frequency modulation system in the mobile radio system. In this event, the digital and the frequency modulation systems are assigned to adjacent ones of the service areas, respectively. The adjacent service areas will be called hereinunder a main and a supplementary service area, respectively.
In the mobile radio system, it is assumed as a particular case that the terminal station moves between the main and the supplementary service areas. In the particular case, a change must be carried out between the digital modulation system and the frequency modulation system when the terminal station moves between the main and the supplementary service areas. The change will be called hereinunder a system change.
In the manner known in the art, it will be assumed that the communication is momentarily interrupted when the system change is carried out. In other words, the communication has a momentary interruption when the terminal station moves between the main and the supplementary service area.
In order to reduce a time of the momentary interruption, a prior art example is disclosed in Japanese Patent Prepublication (Kokai or Publication of Unexamined Patent Application) No. 161427/1983. In the prior art example, the system change is automatically carried out when the terminal station moves between the main and the supplementary service areas. However, it is impossible to eliminate the momentary interruption.